Modelling resistance management benefits of diversity within a fungicidal mode of action with incomplete cross-resistance: the azoles example

The use of mixtures and alternation of fungicides with different mode-of-action (MoA) is well established for fungicide resistance management. The emphasis placed on mixing/alternating fungicides with different MoAs is based on the general absence of cross-resistance between MoAs. Mixtures of fungicides with the same MoA and complete cross-resistance do not contribute to resistance management. However, in some cases cross-resistance is only partial within a MoA, for example fungicides in the demethylation inhibitor (DMI) group. We developed and used a mathematical model to explore possible resistance management benefits of mixtures, alternations and mosaics (where different active substances are applied to different fields within a landscape) of fungicides with the same MoA when cross-resistance is only partial. The deployment of DMI fungicides against Zymoseptoria tritici was used as an important example. Modelling selection for known Z. tritici CYP51 haplotypes, we show that greater diversity of fungicides with incomplete cross-resistance within a MoA can contribute to resistance management, whether deployed in mixture, alternation or mosaic. Combinations of products with low or negative cross-resistance provide the greatest resistance management benefits. The optimal method of deployment of partial cross-resistance was found to vary between years, depending on exact product combinations, pathogen haplotype frequencies and required level of disease control. We consider options for exploiting within-MoA diversity and the extent to which it is beneficial to have several active substances available within a MoA.